Background/Question/Methods   Within most free-living species exists a cryptic yet dynamic community of interacting parasites. Although epidemiological research generally focuses on single-host, single-pathogen systems, growing evidence indicates that interactions among parasite species influences host pathology, parasite transmission, and host-parasite coevolution. Unfortunately, however, an understanding of the mechanisms responsible for parasite coinfection patterns is often precluded by a lack of experimental data. By combining multi-scale field data with manipulative experimental data, we sought to evaluate parasite coinfection patterns and identify their underlying mechanisms. We focused on coninfections by two pathogenic trematodes (Ribeiroia ondatrae and Echinostoma trivolvis) within amphibian hosts.

Results/Conclusions   Field surveys of 86 wetlands and 619 amphibians revealed that the parasites correlated positively at multiple spatial scales, including co-occurrence across the landscape, average abundance among wetlands, and infection intensity within individual hosts. To understand the mechanisms involved and examine how parasites interact within a host, we conducted an exposure experiment in which tadpoles were exposed to one or both parasites at early or late developmental stages. We included two levels of Ribeiroia and three levels of Echinostoma exposure and assessed the individual and combined effects of the parasites on host pathology and parasite recovery. Ribeiroia exposure consistently caused greater pathology than did Echinostoma, including a high frequency of mortality (24%) and malformations (79%). While no interactive effects were noted for host pathology, both parasites decreased the per capita persistence of one another by 17-36%. Thus, in spite of consistently positive relationships from field data, our experiments suggest that parasites exert indirect negative effects upon one another via the immune system (apparent competition), decreasing per capita infection success and emphasizing the importance of parasite community structure. Nevertheless, by delaying time to metamorphosis and reducing behavioral resistance, each parasite could cause an increase in the within-host population of the other in natural systems. These results indicate that understanding parasite interactions depends critically on the scale under consideration, highlighting the complexity of interactions between parasites and the importance of combining field studies of parasite coinfection with mechanistic studies of parasite interactions. In particular, differences as a function of spatial scale and between per capita and population-level process may often obscure a clear understanding of parasite interactions.